In modern high-speed looms using rotary dobbies each heddle frame has a crank arm operated by a rotating cam, which itself controls an adjustment lever known as the main lever, transmitting motion either directly or indirectly via transmission levers to the members which raise said heddle frames in the loom. In such high-speed looms generally up to twenty frames are operated with a standardized 12 mm axial pitch and lifts, which increase starting from the frame closest to the fabric. This variation in lift is obtained by forming curved adjustment guides on the main levers which are stacked one against the other at a pitch of 12 mm on a shaft passing through their central fulcrum, then fixing adjustable sliders on said guides at different distances from the fixed central fulcrum of said main levers. The hinge for the control rods for the lever mechanisms which operate the frames is provided on said adjustable sliders, the desired adjustment in the stroke or lift of the frames therefore being obtained by this variation in the stroke of said control rods.
Inertial forces of the order of some hundreds of kg produced by the movement of the frames, ie where the movements are a maximum, plus the inertial forces of the operating levers all act on the crank arms where the movements are a minimum, to thus produce maximum force, this being of the order of 1000 kg, with the result that the main coupling pins between the crank arms and the relative main levers are the most stressed components of the entire linkage. To this must be added the difficulty of constructing such a main pin because of the standardized 12 mm axial pitch which means the coupling pins must have a length not exceeding 12 mm. Finally, as the force acting on the main pin increases as the cube of the rotational speed of the loom, it is apparent that such main pins are the most critical point of high-speed rotary dobbies, and are able to affect and limit the operating speed of the dobby and hence of the loom.
In the current state of the art relating to high-speed rotary dobbies, the crank arms are generally constructed of solid steel of 6 mm thickness and the main levers, stacked together as stated at a pitch of 12 mm, are also of solid steel of thickness slightly less than said maximum axial dimension of 12 mm to prevent harmful mutual rubbing during operation. A cavity forming two cheeks with a thickness of about 2.5 mm at about 6 mm apart is machined into each of said main levers so that they can receive between them the big end of the relative crank arm which is pivoted thereat by a main through coupling pin passing through corresponding holes in said cheeks and through the central hole of a bearing totally filled with rollers, or of a bush, which had been previously inserted into its housing in the crank arm big end, said main pin then being irremovably fixed in position by clinching it against said two cheeks.
From the aforegoing it is apparent that as said cavity cannot have an axial width greater than about 6 mm without excessively weakening the two cheeks against which the main pin has to discharge the clinching force, said rotation bearing, which acts as an articulated joint between the crank arm and the main lever, cannot have an axial thickness exceeding about 6 mm. The consequence of this limitation is that if, as is currently sought, it is required to increase the loom speed and consequently the speed of its dobby, which for equal frame lifts results in considerably increased loads which, as known, increase with approximately the cube of the speed, the only possible means for increasing the load capacity of the bearing is to increase its diameter, with a consequent equal increase in the entire lever and thus in the size of the entire dobby, resulting in considerably increased dobby space requirements and cost.
A further important drawback of current main levers of solid steel plate is their high cost due to the fact that they have to be completely machined over all their surfaces.
In addition to the cost of contouring the outer profile, which has to be very accurate in the curved guide zone for the slider, and the cost of providing said cavities for housing the big ends of the crank arms, there are also the high costs of the machining required for reducing thicknesses, which have to be different for the various surfaces of the lever. This is because to be able to mount the slider on the main lever, this must be reduced in thickness in its curved slider guide part from 12 mm to about 7 mm, so that the two sides of the two 2 mm thickness cheeks of the slider can pass over it, the slider having a standardized thickness of about 11 mm to prevent any contact with or dragging by the adjacent sliders. Additional costs are due, finally, to the necessity of machining all the other surfaces of the connecting rod to reduce the thickness from 12 mm to 11 mm, still to avoid friction between adjacent levers in operation. In known main levers of the state of the art, the slider can be hinged to the arm driving the levers which operate the frames only at the end facing the loom, whereas in certain applications it would be convenient for this hinging to be at the other end, ie at the dobby end, and in addition the slider is locked in position along the curved guide by press screws acting directly and perpendicularly against the approximately 6 mm edge at the dobby end of the guide, with consequent considerable extension in the direction transverse to said guide, this limiting the travel of the slider along the guide and thus reducing the adjustment range in order to prevent interference between said screws and the crank arms.
Other drawbacks are the impossibility in known main levers of forming their fulcrum zone in a material other than that of the lever body itself, an in particular a material which can be surface-treated to improve its wear resistance, this being very important as this is precisely the zone which supports the thrust of the adjacent levers. In addition, as the two cheeks of the machined cavity in the lever for housing the big end of the crank arm are rigid and cannot be moved elastically apart and moreover cannot be provided with the necessary inner frusto-conical flaring or suitable inner annular recessing, not only do they not allow the use of removable pins such as those described in the prior U.S. Pat. No. 4,770,584 issued on Sept. 13, 1988, by the present applicant for coupling the crank arm to the main lever, and which would allow simple replacement of worn bearings, but they also do not allow the use of bearings with a thickness increased from 6 to 8 mm for higher loads.
A final drawback is that the necessary considerable reduction in the thickness of the lever curved guide from 12 to about 7 mm results in considerable weakening of the zone in which said guide meets the central portion of the lever, which means that as the slider cannot be fixed in proximity to said zone the adjustment range of the slider has to be considerably limited in order to prevent breakage.